1. Field of Invention
The present invention relates to an LED (light-emitting diode) package structure. More particularly, it relates to an LED package structure comprising a photoluminescent diffuser material.
2. Description of the Related Art
As LED luminous efficiency has continued to enhance in recent years, LEDs have gradually replaced fluorescent lamps and incandescent lamps in some applications, such as fast-responding scanner light sources, LCD (Liquid Crystal Display) back light sources, automobile dashboard lighting, traffic lights, and general lighting devices. In comparison with conventional light bulbs, LEDs have absolute predominance because of features, such as compact size, durability, low voltage/current operation, break-resistance, zero heat radiation during illumination, no mercury (and therefore no environmental pollution) and high luminous efficiency (energy saving). In terms of production technologies and applications today, white LED draws the most attention among LEDs' various lighting colors.
White light is a type of light blended from a plurality of colors of light. The white light visible to human eyes comprises at least two colors of light in different wavelengths. For example, blue light and yellow light are blended to form a dual-wavelength white light; or red light, green light and blue light are blended to form a triple-wavelength white light. Currently white LEDs are fabricated in three methods. First, there is a so-called triple-wavelength method, wherein an LED chip set is comprised of a red LED chip, a green LED chip and a blue LED chip. Uniform white light is formed by adjusting respective currents passing through the three chips. This mode features a high luminous efficiency along with a higher production cost. Second, there is a so-called dual-wavelength method, wherein an LED chip set is comprised of a blue LED chip and a yellow LED chip. By adjusting the respective currents of the two chips, uniform white light is formed. This method is characterized in good luminous efficiency and a lower production cost. Additionally, there is a third method, wherein white light is formed by blending blue light formed by a blue LED and yellow light formed by exciting blue light to form yellow phosphor. The third mode features a simpler production process, lower luminous efficiency and a lower cost. Therefore, currently, most white LEDs are based on the third method. Namely, the white light is formed by means of the blue light and the yellow phosphor excited by the blue light.
FIG. 1 is a schematic drawing of a conventional white LED package structure. In FIG. 1, the conventional white LED package structure mainly comprises package lead pins 100, a blue LED chip 102, an inner encapsulant 104 and an outer encapsulant 106, wherein the blue LED chip 102 is disposed on the package lead pins 100 and electrically connected to the package lead pins 100 via two soldering wires 108; the inner encapsulant 104 comprises yellow phosphor, covering the blue LED chip 102; an outer encapsulant 106 is used to cover part of the package lead pins 100, the blue LED chip 102 and the inner encapsulant 104. The aforesaid white LED uses the blue light emitted by the blue LED chip 102 to excite the inner encapsulant 104 to form a dual-wavelength white light, which is blended by the blue light and the yellow light.
FIG. 2 is a schematic drawing of another conventional white LED package structure. In comparison with FIG. 1, the major improvement of the white LED is an additional diffusion layer 110 applied to cover the inner encapsulant 104. The diffusion layer 110 comprises transparent glue in which transparent particles or air bubbles are distributed. The transparent particles or air bubbles in the diffusion layer 110 repeatedly refract the light rays, which enables the tone of the blended light to be more uniform.
However, to obtain a better light interfusing effect, the fluorescent powder on the above-described inner encapsulant 104 and the size and distributed density of transparent particles or air bubbles in the diffusion layer 110 must be well matched. Since too many factors can influence the light interfusing effect, practically it is difficult to some extent to produce and control the light interfusing effect.
To get a detailed understanding of the above-described LED package structure, U.S. Pat. No. 5,998,925 and the ROC patent PN 383508 can be used for reference.